MOSFETs are fabricated with a source, a gate, and a drain, with the source and drains typically defined within a semiconductor substrate as defined regions. The source/drain regions are separated by a channel, over which a gate electrode is disposed. The gate electrode is separated from the channel region by a thin layer of gate oxide. The thickness of the gate oxide will determine certain properties of the transistor, and it is typically fabricated as thin as possible. However, as the gate oxide is decreased in thickness, the voltage breakdown of the oxide will decrease. For example, in 3-volt transistors, a voltage in excess of 5 or 6 volts can cause stress on the gate oxide, which will be destructive to the transistor. Therefore, numerous devices have been provided for preventing the gate oxide breakdown voltage of any transistor from exceeding its gate-to-source or gate-to-drain breakdown voltage.
In one application of a MOSFET, a driving circuit is provided for driving an inductive load, such as a motor. When the device is turned off, there will be a resulting back emf that will drive the voltage on the output node to a high voltage. Since the gate of the driving transistor is typically pulled low, this will result in the voltage across the source and gate exceeding the gate-to-source breakdown voltage thereof. To solve this problem, back-to-back zener or diodes are disposed between the gate and source of the transistor. When the voltage rises above the breakdown voltage of the zener, current will be conducted and there will be a voltage limit across the gate and source. However, this current must flow through the zeners and the driving circuitry to ground, thus causing both dissipation in the zener diodes and dissipation in the driving circuitry, which can be excessive for high back emf voltages.